Structure and dynamics of the biphenyl ring torsion in solid neon and argon

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Abstract
The extent of phenyl ring torsional relaxation possible in low temperature frozen solvents has been investigated for biphenyl in solid neon and argon. In neon biphenyl is trapped in a twisted form, while in argon both twisted and planar forms occur. The twisted forms appear to undergo substantial movement in ϑ towards a planar form before excited state luminescence occurs. The planar form in argon undergoes a highly anharmonic, low frequency (∼8 cm−1), large amplitude ground electronic state torsional oscillation. The excited state splitting (571 cm−1) between the two phenyl rings is 177 cm−1 less than the value reported in crystalline biphenyl. The excited state vibronic structure is considerably different from that observed in crystalline biphenyl. The difference between torsional relaxation in soft solids and viscous flowing fluids, as well as the ability of soft solids to store elastic energy of compression, are discussed.